The present invention relates generally to optical disk recording apparatus based on a mark-length recording scheme which irradiates a laser light beam onto a recording surface of an optical disk to form pits therein for recording of desired information. More particularly, the present invention relates an improved optical disk recording apparatus which provides for improved quality of recorded signals in a situation where the recording is performed at a higher recording speed than the normal (non-increased or one-time) recording speed.
Among various known standards for writable optical disks is the CD-WO (CD Write Once) standard that is also commonly known as the xe2x80x9cOrange Bookxe2x80x9d standard. The CD-WO or Orange Book standard prescribes a recording strategy where a laser-light irradiation time period for forming a pit is set to the following length, for both the normal (non-increased or one-time) and double recording speeds, irrespective of a kind of dye material used in the recording layer of the optical disk:
(nxe2x88x921)T+xcex943T
, where xe2x80x9cnTxe2x80x9d represents a length of the pit to be formed with xe2x80x9cnxe2x80x9d being a variable in the range of 3-11, and xe2x80x9c3Txe2x80x9d represents an extra pulse width to be applied to recording of a shortest 3T pit. Further, the xe2x80x9cOrange Book IIxe2x80x9d standard prescribes that an top-power increasing pulse be imparted to an initial part of each pit-forming laser power irradiation period so as to temporarily increase the laser power level to thereby achieve improved signal quality. It is also prescribed that the top-power increasing pulse be 1.5T in pulse width and 20% of the recording laser power in amplitude.
However, through experiments conducted by the inventors of the present invention, it has been found that jitter characteristics in the recording based on the CD-WO standard tend to get worse as the recording speed increase ratio is raised.
It is therefore an object of the present invention to provide an optical disk recording apparatus which can overcome the problems encountered by the conventionally-known techniques and which can record signals on an optical disk with improved quality even when the recording is performed in a high recording speed ratio.
Generally, in recording at a relatively low speed such as the normal (one-time) or double speed, the recording laser power is relatively weak as illustratively shown in FIG. 2A and presents a weak rise at the beginning of each pit-forming period, so that an initial portion of the pit tends to be formed with a blurred outline. Consequently, unevenness would occur in the initial portions of the individual pits, which would result in great jitters in reproduced signals. For this reason, the CD-WO standard attempts to minimize the unwanted jitters by imparting a relatively great top-power increasing pulse to an initial part of each pit-forming period.
However, as the recording speed increases, for example, to the quadruple, six-times or eight-times speed, higher recording laser power is required as shown in FIG. 2B, and such higher recording laser power can effectively reduce the blur in the initial portion of the pits. If the top-power increasing pulse is imparted in the same ratio as in the lower-speed recording, the irradiated laser light beam would become so excessive in the initial part of the pit-forming periods that the heat of the laser light irradiated onto the disk is transmitted backward, i.e., in an opposite direction to the direction in which the recording advances. Such heat transmission would create unwanted variations in the lengths of lands of the optical disk, which have been considered to be a cause of increased jitters.
In view of such inconveniences encountered by the conventional techniques, the present invention provides an optical disk recording apparatus which is capable of recording information onto an optical disk in a variable recording speed increase ratio and which records the information by irradiating a recording laser light beam onto a recording surface of the optical disk with the recording laser light beam set to a top power level for each pit-forming period and to a bottom power level for each land-forming period between pit-forming periods, to thereby form pits and lands on the optical disk based on a mark-length recording scheme. The optical disk recording apparatus of the present invention is characterized by including a control section which performs control to impart a top-power increasing pulse to part of a top power irradiation period of the recording laser light beam to temporarily increase the top power level thereof and also performs, on the top-power increasing pulse, any one or a combination of the following control:
(a) control to decrease a ratio, to a unit pit length, of a width of the top-power increasing pulse as the recording speed increase ratio is raised, for pits of a same length;
(b) control to decrease a ratio, to a difference between the top power level and the bottom power level, of a difference between a peak level of the top-power increasing pulse and the top power level as the recording speed increase ratio is raised, for pits of a same length; and
(c) control to increase a ratio, to a unit pit length, of a delay of a rise of the top-power increasing pulse from a start of the top power irradiation period as the recording speed increase ratio is raised, for pits of a same length.
The present invention arranged in the above-mentioned manner can prevent excessive laser power from being applied to an initial part of a pit-forming period and thus can effectively minimize unwanted jitters, by decreasing either one or both of the width and amplitude of the top-power increasing pulse as the recording speed increase ratio is raised; note that an increase in the top power level by the top-power increasing pulse can be represented by the xe2x80x9cwidth x amplitudexe2x80x9d of the top-power increasing pulse. Further, by delaying the start of impartment, i.e., the rise, of the top-power increasing pulse relative to the start of the top power irradiation period as the recording speed increase ratio is raised, the present invention can reliably prevent the heat by the application of the top-power increasing pulse from being transmitted backward to a preceding land of the disk, thereby minimizing unwanted jitters. Note that further control may be performed to not impart the top-power increasing pulse when the selected recording speed increase ratio is higher than a predetermined value.
In addition, the present invention can not only significantly improve the quality of signals recorded at a high recording speed (i.e., in a high recording speed increase ratio) but also reduce the recording laser power necessary for attaining a predetermined recording depth of pits. FIG. 3 shows pulse waveforms of the recording laser light which are designed to attain a uniform recording depth of pits by variously changing the amplitude of the top-power increasing pulse. In the figure, hatched sections each represent laser light energy used for recording, and they have a substantially same area. As shown, a peak power level Pb of the laser light beam is made lower as the amplitude of the top-power increasing pulse (i.e., Pbxe2x88x92Pt) is decreased. As a consequence, the life of a laser diode used to generate the recording laser light beam in the present invention can be prolonged to a considerable degree, or an inexpensive lower-performance laser diode can be employed.
According to another aspect of the present invention, there is provided an optical disk recording apparatus which is arranged to record information onto an optical disk in a variable or fixed recording speed increase ratio equal to or higher than the six-times recording speed, by irradiating a recording laser light beam onto a recording surface of the optical disk with the recording laser light beam set to a top power level for each pit-forming period and to a bottom power level for each land-forming period between pit-forming periods to thereby form pits and lands on the optical disk based on a mark-length recording scheme. This optical disk recording apparatus is characterized by including a control section which, in recording in the variable or fixed recording speed increase ratio, performs control to keep constant the top power level of the recording laser light beam without imparting a top-power increasing pulse to the top power irradiation period of the recording laser light beam.
The optical disk recording apparatus according to the other aspect of the invention is designed in consideration of the fact that the use of a high recording speed increase ratio can eliminate the need for the top-power increasing pulse, and is tailored to record any information in a variable or fixed recording speed increase ratio equal to or higher than the six-times recording speed without applying the top-power increasing pulse to the top power irradiation period of the recording laser light beam. This arrangement can dispense with all circuits pertaining to the impartment of the top-power increasing pulse, which thereby can significantly reduce the necessary costs of the optical disk recording apparatus.